You keep saying that... "which expert to believe", but she doesn't appear to disbelieve either witness, Bouf. She appears to be taking what she hears from both witnesses at face value. It simply isn't a credibility issue.
It's her interpretation of what she is hearing. That, too, gets deference under the clearly erroneous standard, but the Appellate Court can't just say "It's a credibility issue and we defer to Judge Du". It isn't a credibility issue. Read the brief. It's an interpretation issue.